An example of a radio communications system in which data is communicated using multi-carrier modulated symbols is the Digital Video Broadcasting (DVB) system. The DVB system utilises a modulation scheme known as Coded Orthogonal Frequency Division Multiplexing (COFDM) which can be generally described as providing K narrow band carriers (where K is an integer) and modulating the data in parallel, each carrier communicating a Quadrature Amplitude Modulated (QAM) symbol. Since the data is communicated in parallel on the carriers, the same QAM-symbol may be communicated on each carrier for an extended period, which can be longer than a coherence time of the radio channel. By averaging over the extended period, the QAM-symbols modulated onto each carrier may be recovered in spite of time and frequency selective fading effects, which typically occur on radio channels.
To facilitate detection and recovery of the data at the receiver, the QAM-symbol are modulated onto each of the parallel carriers contemporaneously, so that in combination the modulated carriers form a COFDM symbol. The COFDM symbol therefore comprises a plurality of carriers each of which has been modulated contemporaneously with different QAM data symbols.
In some radio communications systems multi-carrier modulated symbols include pilot carriers, which communicate data-symbols known to the receiver. The pilot carriers provide a phase and timing reference to facilitate the detection and recovery of data at the receiver. For the Terrestrial version of the DVB standard (DVB-T) the multi-carrier symbol includes both Continuous Pilot (CP) carriers which remain at the same relative frequency position in the symbol and Scattered Pilots (SP). The SPs change their relative position in the symbol between successive symbols, providing a facility for estimating the impulse response of the channel via which the COFDM symbols are being communicated.
Although multi-carrier modulation schemes can provide a robust technique for communicating data in the presence of time and frequency selective fading and Gaussian noise, detecting and recovering data in the presence of impulsive burst noise induced at the receiver input presents a technical problem.